1. Field of the Invention
The field of the present invention pertains to diagnostic x-ray imaging equipment, including among other things, real-time x-ray imaging methods and apparatus.
2. Description of Related Art
Real-time x-ray imaging is increasingly being required by medical procedures as therapeutic technologies advance. For example, many electro-physiologic cardiac procedures, peripheral vascular procedures, PTCA procedures (percutaneous transluminal catheter angioplasty), urological procedures, and orthopedic procedures rely on real-time x-ray imaging. In addition, modem medical procedures often require the use of instruments, such as catheters, that are inserted into the human body. These medical procedures often require the ability to discern the exact location of instruments that are inserted within the human body, often in conjunction with an accurate image of the surrounding body through the use of x-ray imaging.
A number of real-time x-ray imaging systems are known. These include fluoroscope-based systems where x-rays are projected into an object to be x-rayed and shadows caused by relatively x-ray opaque matter within the object are displayed on the fluoroscope located on the opposite side of the object from the x-ray source. Scanning x-ray tubes have been known in conjunction with the fluoroscopy art since at least the early 1950s. Moon, Amplifying and Intensifying the Fluoroscopic Image by Means of a Scanning X-ray Tube, Science, Oct. 6, 1950, pp. 389-395.
Another approach to x-ray imaging involves the use of reverse-geometry x-ray imaging systems. In such systems, an x-ray tube is employed in which an electron beam is generated and focussed upon a small spot on a relatively large target assembly, emitting x-ray radiation from that spot. The electron beam is deflected in a scan pattern over the target assembly. A relatively small x-ray detector is placed at a distance from the target assembly of the x-ray tube. The x-ray detector converts x-rays that strike it into an electrical signal indicative of the amount of x-ray flux detected at the detector. One advantage provided by reverse-geometry systems is that the geometry of such systems allows x-rays to be projected at an object from multiple angles without requiring physical relocation of the x-ray tube.
When an object is placed between the x-ray tube and the detector, x-rays are attenuated and/or scattered by the object in proportion to the x-ray density of the object. While the x-ray tube is in scanning mode, the signal from the detector is inversely proportional to the x-ray density of the object. The output signal from the detector can be applied to the z-axis (luminance) input of a video monitor. This signal modulates the brightness of the viewing screen. The x and y inputs to the video monitor can be derived from the signals that effect deflection of the electron beam of the x-ray tube. Therefore, the luminance of a point on the viewing screen is inversely proportional to the absorption of x-rays passing from the source, through particular areas of the object, to the detector.
Medical x-ray systems are usually operated at the lowest possible x-ray exposure level at the entrance of the patient that is consistent with image quality requirements (particularly contrast resolution and spatial resolution requirements for the procedure and the system being used).
Time and area distributions of x-ray flux follow a Poisson distribution and have an associated randomness. The randomness is typically expressed as the standard deviation of the mean flux and equals its square root. The signal-to-noise ratio of an x-ray image under these conditions is equal to the mean flux divided by the square root of the mean flux, i.e., for a mean flux of 100 photons, the noise is +/-10 photons, and the signal-to-noise ratio is 10.
A relatively high level of x-ray flux makes it easier to yield high resolution images. A high level of x-ray flux can create a potentially more accurate image by decreasing the x-ray quantum noise. The x-ray flux should be projected through the object often enough to allow a frame rate (the number of times per second that an object is scanned and the image refreshed) which produces an acceptable image picture and refresh rate at a video display device.
In a reverse-geometry medical imaging system, the desire for high levels of x-ray flux normally requires extended bombardment of an x-ray tube target assembly by a high energy electron beam. In creating x-rays in response to an electron beam, the x-ray target assembly is raised to high temperatures; in some systems, the target assembly material is heated to temperatures in excess of 1000 degrees centigrade. Prolonged exposure of the target assembly to high temperatures can cause melting or cracking of the target assembly material due to thermal stress. Even if the prolonged exposure does not immediately cause the target assembly material to fail, such exposure can cause long-term damage that affects the longevity of the target assembly.
Thus, several conflicting factors, among them image resolution, frame rate, and the thermal qualities of x-ray target assembly materials, may work to limit the usefulness of conventional x-ray imaging systems. Maintaining an electron beam bombardment of an x-ray tube target assembly for a sufficient period of time to satisfy flux/frame rate requirements may result in damage to the target assembly material. However, reducing the flux/frame rate requirement to prevent damage to the target assembly may result in diminished image quality.
Therefore, there is a need for an x-ray imaging method and system that is capable of addressing the shortcomings of the prior approaches. There is a need for a method and system that can provide a high level of x-ray flux to each portion of an object being imaged during a short period of time while preventing damage to the target material and increasing target longevity.